get

Returns the value at a specified date. It returns null
if no value has been set for that date. Thus, it is impossible to
distinguish an unset value and a set value of null. If
such a distinction is necessary, the client can use a special marker
value instead of null to signal a null value.

Parameters:

at - date at which this dated value is to be queried

Returns:

the value at a specified date

Throws:

java.lang.NullPointerException - if at is null

hashCode

Returns a hash code value of this dated object at the specified date.
For two dated objects o1 and o2 and two dates
d1 and d2, if
o1.equals( d1, o2, d2 ) returns true,
then o1.hashCode( d1 )
must equal o2.hashCode( d2 ).